1Medical University of South Carolina, College of Nursing and Medicine, Charleson, SC, 2University of Tulsa, Exercise and Sport Science, Tulsa, OK, 3University of South Carolina, Department of Exercise Science, Division of Health Aspects of Physical Activity, Arnold School of Public Health, Columbia, SC, USA; 4University of Padova, Department of Medicine, Sports Medicine Division, Padova, Italy; 5University of South Carolina, Department of Epidemiology and Biostatistics, Arnold School of Public Health, Columbia, SC, USA

Background: This study evaluated the reliability and criterion validity of the Mywellness Key accelerometer (MWK) using treadmill protocols and indirect calorimetry.Methods: Twenty-five participants completed two four-stage 20-minute treadmill protocols while wearing two MWK accelerometers. Reliability was assessed using raw counts. Validity was assessed by comparing the estimated VO2 calculated from the MWK with values from respiratory gas exchange.Results: Good overall and point estimates of reliability were found for the MWK (all intraclass correlations > 0.93). Generalizability theory coefficients showed lower values for running speed (0.70) versus walking speed (all > 0.84), with the majority of the overall percentage of variability derived from the participant (68%–88% of the total 100%). Acceptable validity was found overall (Pearson’s r = 0.895–0.902, P < 0.0001), with an overall mean absolute error of 16.22% and a coefficient of variance of 16.92%. Bland-Altman plots showed an overestimation of energy expenditure during the running speed, but total kilocalories were underestimated during the protocol by approximately 10%.Conclusion: Good validity was found during light and moderate walking, while running was slightly overestimated. The MWK may be useful for clinicians and researchers interested in promotion or assessment of physical activity.